US 20050104684 A1
A planar integrated circuit includes a semiconductor substrate having a substrate surface and a trench in the substrate, a waveguide medium in the trench having a top surface and a light propagation axis, the trench having a sufficient depth for the waveguide medium to be at or below said substrate surface, and at least one Schottky barrier electrode formed on the top surface of said waveguide medium and defining a Schottky barrier detector consisting of the electrode and the portion of the waveguide medium underlying the Schottky barrier electrode, at least the underlying portion of the waveguide medium being a semiconductor and defining an electrode-semiconductor interface parallel to the light propagation axis so that light of a predetermined wavelength from said waveguide medium propagates along the interface as a plasmon-polariton wave.
1. A planar integrated circuit, comprising:
a semiconductor substrate having a substrate surface;
a waveguide buried in said substrate and comprising a core and a waveguide confiner around said core, said waveguide confiner including a ceiling;
an electrode in said ceiling and contacting said core to form an interface between said electrode and said core, said interface defining a detector portion of said core, said detector portion of said core and said electrode comprising a Schottky barrier detector buried in said substrate.
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plural transistors formed in or over said substrate surface, each of said transistors comprising a source, a drain and a gate;
an insulator layer overlying said waveguide and said plural transistors, said insulator layer having plural contact openings to respective ones of said source, drain and gate of respective ones of said transistors, to said electrode and to one of: (a) said semiconductor waveguide core, (b) said waveguide confiner;
a conductive interconnect layer formed over said insulator layer and comprising conductive contacts in said contact openings and conductive lines on said planar surface of said insulator layer connected to respective ones of said conductive contacts, at least one of said conductive lines forming a connection between said electrode and one of said source, drain or gate of one of said transistors.
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an optical clock signal generator having an optical clock signal output coupled to said waveguide core.
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20. An integrated circuit, comprising:
a semiconductor substrate having a substrate surface;
a waveguide medium in said substrate and having a top surface and a light propagation axis;
at least one Schottky barrier electrode formed on said top surface of said waveguide medium and defining a Schottky barrier detector comprising said electrode and a portion of said waveguide medium underlying said Schottky barrier electrode, at least said portion of said waveguide medium being a semiconductor and defining an electrode-semiconductor interface parallel to said light propagation axis.
21. The apparatus of
plural transistors formed in or over said substrate surface, each of said transistors comprising a source, a drain and a gate;
an insulator layer covering said substrate surface and overlying said waveguide medium and said plural transistors, said insulator layer having plural contact openings to respective ones of said source, drain and gate of respective ones of said transistors, to said electrode and to said portion of said waveguide medium;
a conductive interconnect layer formed over said insulator layer and comprising conductive contacts in said contact openings and conductive lines on said insulator layer connected to respective ones of said conductive contacts, at least one of said conductive lines forming a connection between said electrode and one of said source, drain or gate of one of said transistors.
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plural Schottky barrier electrodes formed on said top surface of said waveguide medium at spaced apart intervals along a length of said waveguide medium in said light propagation direction, and defining respective plural Schottky barrier detectors comprising the respective plural Schottky barrier electrodes and the respective portions of said waveguide medium underlying said respective Schottky barrier electrodes at said spaced apart intervals, and further defining intervening sections of said waveguide medium between said respective portions underlying said Schottky barrier electrodes.
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an optical clock signal generator having an optical clock signal output coupled to said waveguide medium.
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40. In an integrated circuit comprising a semiconductor substrate and plural transistors formed in or over said substrate, a method of propagating a signal to a set of said plural transistors, comprising:
providing a buried optical waveguide core in said substrate that extends near respective ones of said set of plural transistors;
providing plural Schottky barrier detectors buried in said substrate as respective electrodes contacting respective top portions of said waveguide core near the respective ones of said set of plural transistors;
coupling respective ones of said electrodes to respective ones of said set of plural transistors;
propagating a light beam modulated with said signal through said waveguide core.
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This application claims priority of U.S. Provisional Application Ser. No. 60/508,421, filed Oct. 3, 2003 entitled A PLANAR INTEGRATED CIRCUIT INCLUDING A PLASMON WAVEGUIDE-FED SCHOTTKY BARRIER DETECTOR AND TRANSISTORS CONNECTED THEREWITH by Gregory Wojcik, et al. and assigned to the present assignee.
This application contains subject matter related to U.S. application Ser. No. 10/280,492 filed Oct. 25, 2002 entitled OPTICAL READY WAFERS by Lawrence West et al. and assigned to the present assignee, and U.S. application Ser. No. 10/280,505 filed Oct. 25, 2002 entitled OPTICAL READY SUBSTRATES by Claes Bjorkman, et al. and assigned to the present assignee.
Schottky barrier detectors are used as optical detectors in night vision applications, for example, in which photons in the far infrared range, typically in the 10 micron wavelength region, are detected. Typically, a semiconductor such as silicon is employed with an overlying Schottky electrode of a metal material chosen to provide a very low (0.1-0.2 ev) Schottky barrier energy across the metal-semiconductor Schottky interface. In a Schottky detector, the incoming light is incident on the overlying metal Schottky electrode, in a direction generally normal to the electrode surface. Electrons in the metal electrode absorb incident photons and are thereby given sufficient energy to cross the Schottky energy barrier (between the metal and semiconductor layers) and flow into the semiconductor layer, resulting in a photo current across the device.
Schottky barrier detectors have not been generally employed in optical communication systems, which tend to operate at shorter wavelengths in the range of about 1 to 3 microns. This is because a Schottky detector is typically less efficient than other types of detectors in this wavelength range. And, detector efficiency in a long-range optical communication system is critical.
The efficiency of a Schottky barrier detector is determined by a number of factors, including the geometry of the electrode or metal layer. This is because the incident light, generally normal to the electrode surface, is absorbed by electrons during its passage through the thin metal layer. The probability of absorbing all incident photons is enhanced in a thicker electrode layer. But, as the metal electrode layer thickness is increased, the number of photo-excited electrons that actually reach the semiconductor-metal interface is reduced by collision losses within the metal electrode. Thus, optimizing photon absorption in the Schottky electrode is difficult and depends upon controlling the very small dimension of the Schottky electrode thickness.
In summary, it has not seemed practical to use a Schottky detector for optical communications because high detector efficiency is so important, and because controlling detector efficiency requires difficult or expensive process control over the Schottky electrode thickness.
The invention that will be described below in this specification arose during consideration by the inventors herein of the problem of integrating an optical waveguide and an optical detector in a planar integrated circuit as a locally distributed optical communication system operating at a short wavelength (e.g., 1.3 microns). A locally distributed optical communication system envisioned by the inventors herein could distribute a global clock signal in a VLSI integrated circuit (for example) via a network of optical waveguides, to reduce power consumption associated with conventional electrical distribution of the clock signal. One problem is that in such a system, the waveguide would be complex (e.g., it might need a number of terminations equal to the number of detectors to be fed by the waveguide). A solution needed to be found that would provide a very simple and easily fabricated waveguide/detector structure.
One problem with a conventional Schottky barrier detector integrated with a waveguide, such as that described in U.S. Pat. No. 4,857,973, is that its structure cannot be easily combined in a planar integrated circuit structure. This is because such a device is not planar, in that the waveguide is a rib above the plane of the integrated circuit. This feature is necessary in order to establish a difference in refractive index around the rib that confines light travelling through the rib. Such a structure cannot be combined in a planar VLSI integrated circuit because the fabrication and performance of such a circuit requires a planar structure. In a planar structure, structural features in the semiconductor substrate are below the top surface of the substrate, the surface of the substrate being reserved for overlying insulator layers and such features as conductor lines, and, in some cases, silicon-on-insulator transistor structures, for example.
The problem is how to integrate a Schottky barrier detector with a waveguide in a planar VLSI integrated circuit.
A waveguide medium and Schottky barrier detector integrated with the waveguide medium are buried inside a semiconductor substrate of a planar integrated circuit, so that the integrated waveguide and Schottky barrier detector are generally below or co-planar with the plane of the substrate, thus providing a planar integrated circuit structure. The waveguide medium can be surrounded by a wall having an index of refraction different from that of the waveguide medium. The wall can be separately formed or may be the substrate itself. The Schottky barrier detector is connected to transistors in the planar integrated circuit via overlying conductor layers of the integrated circuit. The waveguide feeds a light beam to be detected to a Schottky barrier detector parallel to and in the plane of the electrode-semiconductor interface of the Schottky barrier detector. Optical absorption of the Schottky detector is determined by the length of the electrode along the direction of the light beam, which can be easily controlled to a high degree of accuracy by conventional photolithography, a significant advantage.
One feature of this structure is that the light propagates at least in part as plasmon-polariton waves along the electrode-semiconductor interface. A plasmon-polariton (surface plasmon) is an interface wave whose distribution has evanescent tails on each side of the interface. An advantage is that such plasmon-polariton waves increase the amount of light near the metal-semiconductor interface (where the light is most likely to be absorbed and contribute to the photocurrent of the Schottky detector). This enhances the Schottky barrier detector efficiency.
A structure integrating the buried Schottky barrier detector described above and a buried waveguide is realized in a planar integrated circuit structure by forming a Schottky barrier electrode directly on (contacting) a portion of the buried waveguide. In one example, plural Schottky barrier optical detectors are provided at spaced intervals along the length of the buried waveguide by forming respective metal electrodes over successive portions of the waveguide. The various lengths of the Schottky electrodes along the direction of the waveguide may be adjusted to provide suitable apportionment of the light among the plural Schottky barrier detectors. The waveguide media is a semiconductor, either in those portions of the waveguide underlying the successive Schottky barrier electrodes, or throughout the entire waveguide.
The ceiling layer 50 has a window 50 a in which a metal layer 120 is deposited to form a buried Schottky barrier detector 100 integrated with the buried waveguide 14. All the features of the integrated waveguide-detector structure 14, 100 are at or below the plane of the top surface of the substrate 10. In the illustrated embodiment, the waveguide-detector structure 14 is buried well below the top surface of the substrate 10. An advantage is that the waveguide-detector structure 14, 100 is completely integrated with the planar integrated circuit so that the waveguide-detector structure 14, 100 does not interfere with other structures or layers overlying the substrate 10 and may be connected to other devices in the integrated circuit via the overlaid conductor structures provided in such an integrated circuit. While the core 110 is illustrated in
The top surface of the waveguide-confining ceiling 50 is below (or, alternatively, co-planar with) the top surface of the semiconductor substrate 10. For this purpose, the waveguide 14 is buried in an elongate trench formed in the top surface of the semiconductor substrate 10. This preserves the plane of the top surface of the semiconductor substrate 10 for deposition of other features, such transistor devices and an insulator layer. For example, a field oxide layer 60 (formed of silicon dioxide, for example) covers the entire structure. Transistors 12 are formed in or on the semiconductor substrate 10 in the conventional manner. The transistors may be formed by implanting source and drain regions in the substrate 10 and forming insulated polysilicon gates over the source-to-drain channels, in the well-known manner. Alternatively, the transistors 12 may be formed as silicon-on-insulator devices by forming semiconductor islands on an oxide layer over the substrate 10 for connection to the Schottky barrier detector, as will be described later in this specification. In such a case, the transistors are well above the surface of the semiconductor substrate 10.
The Schottky barrier detector 100 consists of the semiconductor layer 110 and the overlying metal layer or electrode 120. The electrode contacts the top surface of the semiconductor layer 110 to form a planar interface 115 therebetween. An incoming light beam 130 is guided in a direction parallel to the planar interface 115 and also parallel to the length L of the electrode 120. The incoming light beam 130 is incident on and normal to an imaginary planar edge face 132 formed by imaginary co-planar side edge faces 110 a, 120 a of the semiconductor layer 110 and the electrode 120, respectively. The photons of the incoming light beam 130 couple to a plasmon-polariton mode near the interface 115, so that the light propagates at least in part along the plane of the interface 115 as a plasmon-polariton wave. The plasmon-polaritons thus created are confined about the interface 115 in a thin region 140 that extends along at least most of the length L of the electrode 120. The direction of light propagation is parallel to the electrode length L. The confined distribution of the plasmon-polariton electric field within the region 140 is illustrated in the graph of
The integrated circuit includes plural transistors 12, one of which is visible in the cross-sectional view of
As shown in
A dominating factor in determining the fraction of the photons absorbed in the Schottky detector 100 is the length L of the electrode 120. Increasing the length L increases the surface interaction with the light beam at the interface 115, thereby increasing the fraction of the incoming photons that are absorbed into electrode 120 and thus increasing the resulting photocurrent. The electrode length L may be chosen to set the fraction of absorbed photons to any desired fraction or percentage, up to nearly 100%. This fraction may be accurately determined using conventional photolithographic techniques to appropriately set the length L of the electrode 120. It is limited, however, by optical propagation losses inherent in the plasmon-polariton mode (through the region 140).
The electrode thickness T should exceed the skin depth of the metal electrode 120 at the wavelength of the light beam 130 in order to ensure some photon absorption (by preventing loss of photons by transmission through the electrode 120). However, the electrode thickness T should be less than about half the electron mean free path length in the metal electrode 120 avoid electron collision losses within the electrode 120 that would otherwise prevent photo-excited electrons in the electrode 120 from reaching the interface 115 and crossing into the semiconductor layer 110. For example, if the electrode 120 is aluminum and the semiconductor layer 110 is silicon and the wavelength of the light beam 130 is 1.3 microns, then the skin depth is about 50 nm and half the electron mean free path length is about 1 micron, and T should lie between these two values. The electrode thickness may be selected to be on the low side of this range (e.g., T=2 nm) so that the electrode 120 presents less of an optical discontinuity in the waveguide 110. For example, the electrode thickness T may be between about 1 and 2 nm while the thickness of the core 110 may be between about 1 and 2 microns.
The light beam 130 incident at the interface 115 of the detector 100 of
The ceiling layer 450 divided along its length into successive sections (e.g., sections 450 a, 450 b), separated by openings or spaces through which successive Schottky barrier electrodes are deposited on the semiconductor core 405. For example, the sections 450 a, 450 b are separated by an opening or space of length L, this space permitting deposition of a thin metal Schottky barrier electrode 460 of length L on the top surface of the semiconductive core or medium 405. The opening or space of length L is then refilled with the same type of material that was removed to form the opening. The electrode 460 corresponds to the Schottky barrier electrode 120 of
The next space between successive sections of the ceiling 450 permits deposition of a second thin Schottky barrier electrode 470 on the semiconductor core 405. This forms a second Schottky barrier detector 475 consisting of the second electrode 470 and the region of the semiconductor core or medium 405 directly underlying the second electrode 470.
Depending upon the electrode length L of the first Schottky barrier electrode 460, only a fraction of the incoming light beam 481 (e.g., about 10%) may be absorbed or lost in the first Schottky barrier detector 465, the remaining fraction being carried downstream through the waveguide 400 (with effectively no losses) as a residual light beam 482 for detection/absorption in the second Schottky barrier detector 475. The waveguide medium 405 may be continuous as illustrated in the drawings of
The waveguide walls 420, 430, 440, 450 are illustrated as being continuous along the entire length of the dielectric medium 405, except where the ceiling 450 has been temporarily opened to accommodate the deposition of each Schottky barrier electrode 120. Once the Schottky barrier electrodes 120 are deposited on the core 405, the openings may be re-filled.
Each Schottky barrier detector 465, 475 integrated with the waveguide 400 is generally the same as the Schottky barrier detector 100 of
Generally, in the waveguide 400, the region of maximum intensity is the axial center of the core 405, corresponding to the peak “A” of
In order to reduce reflection losses at the edge of the electrode 460 protruding into the core 405, the electrode 460 may have a wedge shape in the horizontal plane, as illustrated in
As shown in
The light conducted by the waveguide 400 of
The integrated waveguide/detector structure of
The waveguide core layer 814 has a U-shaped cross-section, as shown in
While the invention has been described with respect to an embodiment in which the waveguide core has a higher index of refraction than the surrounding wall, in some cases it may be possible to reverse this relationship while carrying out the invention. Moreover, the waveguide sections between successive Schottky barrier detectors has been described as having a core surrounded by a wall (either the substrate or a separately formed wall), the core in such sections being either a semiconductor or a dielectric. However, the waveguide in the sections between Schottky barrier detectors can take other suitable forms capable of supporting an optical mode. For example, in those sections between Schottky barrier detectors, the core may contain a solid material or it may be empty, while the surrounding wall may be a semiconductor, a dielectric or a conductor, for example.
While the invention has been described in detail by specific reference to preferred embodiments, it is understood that variations and modifications thereof may be made without departing from the true spirit and scope of the invention.